The present invention relates to magnetic flowmeters of the type used to measure flow of process fluid through process piping. More specifically, the present invention relates to saturation of electronics used to measure such flow.
Field devices are used in industrial process monitoring and/or control systems to monitor process variables associated with a particular process. Such process variables can include fluid pressure, fluid flow rate, fluid temperature, level, etc.
Magnetic flow meters are a type of field device that are used to measure a fluid flow rate of a conductive process fluid as it flows within a flow tube that is coupled to a pipe. A particular magnetic flow meter includes an electromagnet coil and electrodes. In accordance with Faraday's law of electromagnetic induction, the electromagnet coil is used apply a magnetic field to the process fluid within the flow tube. The applied magnetic field and movement of the fluid induces an electromotive force (EMF) in the process fluid, which is proportional to the flow rate. The electrodes are positioned in the flow tube to make electrical contact with the flowing process fluid to sense the induced EMF. In a particular embodiment, the EMF is measured by the flow meter using an amplifier connected to the electrodes to amplify the EMF signal, and an analog-to-digital converter (ADC) to quantize the output of the amplifier to produce a data value related to the fluid flow rate.
During operation of the magnetic flowmeter, there are a number of conditions which can cause the flowmeter to incorrectly provide an output indicating that there is no flow. One such condition is caused by saturation of the circuitry coupled to the electrodes. Saturation occurs when a signal is applied to an input of an electrical component which exceeds a maximum level for the component. In a magnetic flowmeter, when saturation occurs in the circuitry coupled to the electrodes, a condition may arise in which measurement circuitry incorrectly interprets the measured signal to have a value indicating that there is no flow. This is a type of “on scale” error. The saturation of the circuitry is difficult to detect and can lead to incorrect flow measurements. Issues related to such saturation are discussed in U.S. Pat. No. 7,921,734 to Foss et al, issued Apr. 12, 2011, entitled SYSTEM TO DETECT POOR PROCESS GROUND CONNECTIONS and commonly assigned herewith.